This invention relates in a general aspect to a continuous process for treating a solid material during submersion in a body of liquid, and in another aspect to a method for making foam sheets by a process wherein foamable thermoplastic resinous sheets or preforms are continuously foamed by submersion in a body of heated liquid.
It is known in the art to produce thermoplastic resin foam in sheet form by compounding such a resin, e.g., polyethylene, with a thermally-decomposable blowing agent, e.g., azodicarbonamide; heat-plastifying the composition under conditions not causing decomposition of the blowing agent and fabricating therefrom a non-cellular sheet and crosslinking the resulting sheet, e.g, by high energy electron beam irradiation or chemical crosslinking means, to obtain a crosslinked foamable sheet. Thereafter, the resulting crosslinked foamable sheet is optionally preheated to temperatures below the decomposition temperature of the blowing agent and is then heated to effect decomposition of the blowing agent and formation of gas-filled predominately closed cells in a resulting foamed resin sheet product. However, in the aforementioned type of foaming process, it is, by virtue of the relatively low thermal conductivity of thermoplastic resins (and especially of closed cell foams thereof), generally difficult to heat the crosslinked sheet product uniformly throughout to obtain uniform foaming, particularly when the heating/foaming operation is conducted in a continuous manner. Moreover, since the foaming sheet is expanding in all directions and since the resin is in heat-softened condition when being foamed, it is generally desirable that said foaming sheet be supported in some way during said foaming operation.
There are several methods which have been suggested to avoid the above-noted problems which are encountered while foaming foamable resin preforms, but in the attempts made to date other problems have arisen making such methods far from ideal.
In U.S. Pat. No. 3,562,367 to Shinohara et al., there is described a float process of foaming foamable crosslinked olefinic resin sheet. In that process, such a sheet is heated in part and supported by floating it on the surface of a liquid which has a density greater than that of both the unfoamed sheet and the foamed sheet prepared therefrom and which is maintained at a temperature suitable for effecting the foaming. In such process, the sheet is also concurrently heated from above by a radiant heat source. A suitable heat transfer liquid is a molten mixture of low melting salts, such as a mixture comprising potassium nitrate and sodium nitrite and nitrate, which melts at a temperature below the foaming temperature of the foamable sheet and which is used at a temperature that softens the sheet and decomposes the blowing agent. In such foaming step, the sheet is supported on the flat upper surface of the heat transfer liquid and is free to expand in all directions as the sheet foams while so supported. However, the heating of the foamable sheet to effect foaming thereof by heat transferred from the liquid beneath the sheet to the underneath side thereof must be exactly matched by heat transferred to the upper side of the sheet by the radiant heating sources above the sheet, e.g., infrared lamps or like devices. Attaining such matching is particularly difficult in continuous operation where the foamable sheet continuously moves across the surface of the heat transfer liquid bath, foams, and is continuously taken off and away as foamed sheet, wherein the temperature of the sheet changes during such transit and the temperature profiles of the sheet may also change during the operation. Moreover, sheets of some resins, such as radiation crosslinked linear polyethylene, which are susceptible to degradation and other deleterious effects of exposure to air at high temperatures, cannot consistently be foamed by the liquid bath surface float method without elaborate and inconvenient measures to maintain an appropriately inert atmosphere over the liquid bath.
In U.S. Pat. No. 4,143,106 to Coyne, there is described a method for foaming a foamable thermoplastic resin preform by floating such preform upon a heated liquid bath while heating it from above by flooding the upper surface of the floating preform with a blanket of liquid at substantially the same temperature as that in the heated bath. This process helps reduce the amount of the heated foam surface in contact with air but typically does not entirely eliminate such contact with air. Moreover, other problems such as non-uniform heating can still be encountered.
In U.S. Pat. No. 4,155,965 to Allada, there is described a process for heat foaming foamable preforms by totally submerging them in a heated liquid bath. In such process, the foaming preform is kept submerged under a fast moving endless belt which generates a dynamic fluid boundary layer, which prevents direct contact between the foam and the belt and which thereby allows transport of the foam while it is sticky and has little tensile strength. Under some conditions, however, such as when the belt speed is much higher than the foam transport speed and the temperature is high, the shear stress exerted by the belt and the dynamic fluid layer can stretch the soft expanding sheet undesirably. In addition, the machinery and moving parts required for this process complicate its use.
In addition, the general handling problems caused by the stickiness and low tensile strength of the hot foaming resin are similar to problems which are encountered in many other process operations where a soft solid sheet or strip is submerged in a treating liquid of greater density under conditions that the solid is sticky and/or has insufficient tensile strength to withstand the stretching forces involved in handling and/or conveying the resin preform.
In view of the foregoing, it would be desirable to provide an improved method and means for carrying out such method for treating solid materials which are sticky and/or have low tensile strength. Particularly, it would be desirable to provide method and means to effect foaming of a heat foamable material preform especially in the form of sheet comprising crosslinked thermoplastic resin and foaming agent such as thermally decomposable chemical blowing agent, which method and means would supply the necessary heat uniformly and simultaneously to all sides of the foamable and foaming material while at the same time protecting it from exposure to deleterious atmosphere, and would allow the foaming material to expand freely in all directions without distortions and without undesirable stretching due to shear stresses while carrying out the operation in a continuous and efficient manner.
A general object of this invention is to provide improved method and means for treating solid materials which are sticky and/or have little tensile strength.
A more specific object is to provide method and means to make thermoplastic resin foam, especially in sheet form, from foamable thermoplastic resin composition containing latent foaming agent such as thermally decomposable chemical foaming agent in continuous manner by continuously submerging the foamable material in a body of heat transfer liquid and conducting the material and resulting foaming and foamed product through and out of such body of liquid.
Other objects and advantages of the invention will become evident in the description that follows.